Optical signaling and communication systems are widely used in the art today. The optical signals may be transmitted through fiber optical cables or in free space or using mirrors or lenses or combinations thereof. In a typical system, an electrical signal is converted into an optical signal, transmitted through some media to a receiver and then used directly to modulate the properties of some receiving media and/or reconverted back to an electrical signal for further processing.
An important aspect of such optical communication systems is their linearity, that is, the degree to which the optical output of the transmitter or the electrical output of the receiver resembles the original electrical input signal, taking into account all of the harmonics, intermodulation and distortion products produced within the optical communication system. A significant problem with present day analog optical transmission or communication systems is that they are not particularly linear. This arises in part because they often must be operated at high levels in order to obtain sufficient optical output. As a consequence of the non-linearity, the optical signal can contain a significant amount of distortion and/or spurious signals generated within the optical system. This problem is exacerbated by the need to simultaneously generate or transmit several different information streams at the same time, as for example, by amplitude modulating an optical carrier with several different input signals at different average frequencies or on different sub-carriers.
Non-limiting examples of such spurious distortion products are second order harmonics (i.e., cross-modulation) and third-order harmonics (i.e., intermodulation and triplebeats). If a Fourier transform of the modulated optical signal is examined, these distortion products manifest themselves as spurious side-bands lying outside the modulation base-band around the optical carrier or subcarrier. Thus, there is an ongoing need to reduce the distortion products in analog optical generation and/or transmission systems.